The present invention relates to a method of laser drilling a hole in a workpiece, for example a film cooling hole in a combustor component or a turbine blade or vane.
It is known to provide film cooling of various components of gas turbine engines which are exposed to extreme heat. Such components include the elements which make up the combustion chamber and the turbine rotor blades and nozzle guide vanes, which are situated directly downstream of the combustion chamber. The components to be cooled are generally provided with film cooling (effusion) holes arranged in rows such that streams of cooling air may emerge from the holes onto the surfaces of the components. The resulting films of air form barriers against the hot gases and prevent them from melting the components. The effusion holes may be angled obliquely relative to the surfaces of the components, this tending to produce a relatively smooth film of air.
It is known to form effusion holes in gas turbine engine components by laser drilling. Laser drilling involves the use of a laser machine tool to focus a beam of high-energy coherent radiation (laser light) on to the workpiece surface, hence causing material to be vaporised and expelled. Laser drilling has two variants: trepanning and percussion drilling, and this invention relates particularly to trepanning.
In trepanning, a continuous or pulsed laser beam is focused generally on the surface of the workpiece to drill through the workpiece at one point. The laser beam is subsequently moved through a circular path to cut the circumference of a cylindrical hole. Laser trepanning is used to drill holes with diameters greater than about 0.3 mm. Laser trepanning produces a more consistent and a better quality hole than percussion laser drilling, but is slower.
It is known to provide gas turbine engine components with thermal barrier coatings to enable the components to operate at high temperatures or to operate at conventional temperatures for longer periods. However, such coatings can make the laser drilling of cooling holes more difficult.
Attempts to increase the rate of laser drilling of angled effusion holes in combustor components have involved increasing the pulse energy of the laser and the feed rate. However, it has been found that this may cause the thermal barrier coating to delaminate.
According to the invention there is provided a method of producing an angled hole in a workpiece using a laser beam, the workpiece comprising a surface, the surface defines an angled hole, the angled hole having a first edge region and an opposite edge region, the first edge region of the angled hole meeting a surface of the workpiece at an obtuse angle and the opposite edge region of the angled hole meeting the surface of the workpiece at an acute angle, the method including the steps of: using the laser beam to drill an initial orifice in the workpiece; translating the laser beam away from the initial orifice to a starting point at a periphery of the hole to be produced; and moving the laser beam such that it traces a closed loop path on the surface of the workpiece, thus defining the periphery of the hole, whilst maintaining the focus of the laser substantially on the surface of the workpiece; wherein the starting point is located in the edge region of the angled hole which meets the surface of the workpiece at an obtuse angle.
Preferably the closed loop is traced so as to surround the initial orifice.
Preferably the closed loop path is elliptical.
Preferably wherein the laser beam is angled obliquely relative to the surface of the workpiece.
Preferably the laser beam is oriented at substantially the same angle as the hole to be produced.
Preferably the laser beam is guided through a laser nozzle which is movable to control the position and/or focus of the laser beam. Preferably the laser nozzle comprises a tip and a central axis, the movement of the nozzle is controlled with reference to three mutually perpendicular axes defined at the tip of the nozzle, one of the axes being parallel to the central axis of the hole to be produced and the other two axes defining a plane perpendicular to the hole to be produced, and wherein movement of the nozzle has a component consisting of a circle traced on the plane and a component consisting of variable movement along the axis perpendicular to the plane, thereby tracing an elliptical path substantially on the surface of the workpiece.